select top 1 with ties
cliente, vendedor, sum(IMPORTE)
from
t1
group by
cliente, vendedor
order by
sum(IMPORTE) desc

select top 1 with ties
cliente, vendedor, sum(IMPORTE)
from
t1
group by
cliente, vendedor
having
sum(IMPORTE) = (
select max(sum_importe)
from (
select sum(IMPORTE) as sum_importe from t1 group by cliente, vendedor
) as t
)
go